372 G. O. Batzli et al. 



lapses and the lemming population is concentrated into small areas of 

 suitable habitat; when populations are dense, a period of intense social 

 interaction may ensue. Lemmings may be forced into unfamiliar habitat 

 where they may become more susceptible to predation. Thus, spring is a 

 particularly traumatic period during which breeding subsides and mor- 

 tality is high. In years of high rainfall much of the low-lying habitat con- 

 tinues to be flooded and unavailable to lemmings most of the summer. 

 Polygon troughs and wet meadows, which are most susceptible to flood- 

 ing, have the highest density of food plants and are preferred winter hab- 

 itat. If these areas are flooded at freeze-up, they can remain unavailable 

 to lemmings all winter long. The effect of late-season flooding may be 

 greater than its spatial extent alone would suggest. 



In August and early September lemmings encounter the only 

 marked diurnal change in light intensity and temperature that they exper- 

 ience. The snowpack generally develops between mid-September and 

 mid-October; delay poses two dangers for lemmings. First, if the vegeta- 

 tion becomes coated by freezing rains, the energy cost of foraging greatly 

 increases. Second, in the absence of snow the ground surface is exposed 

 to increasingly low nighttime temperatures. The reduction of breeding at 

 this time attests to the severity of the period in the annual cycle of the 

 lemming. Fuller (1967) has suggested that weather conditions in autumn 

 can be a major factor influencing the population dynamics of lemmings. 



The dense and shallow snow cover offers only modest protection 

 from winter cold; very low temperatures are encountered at the ground/ 

 snow interface. In 1970-71 the temperature at the ground surface under 

 30 cm of snow at the Carex-Oncophorus meadow dropped below -20 °C 

 in early December and to -25 °C in early March (Chapter 2). In March 

 1972 the mean temperature was -26 °C under 29 cm of snow (MacLean et 

 al. 1974). Temperatures as low as -32 °C were recorded under 15 cm of 

 snow. 



Lemmings apparently take advantage of the higher temperatures 

 that are found under deeper snow by concentrating their activity in poly- 

 gon troughs. MacLean et al. (1974) found a significant correlation be- 

 tween nest density and amount of topographic relief {r = 0.52, p < 0.01), 

 and presented evidence that winter reproduction was inhibited in years of 

 shallow snow accumulation. 



The structure of the snowpack may be just as important to lemmings 

 as its depth. Once the snowpack accumulates, moisture is redistributed, 

 and a strong structural profile develops (Chapter 2). The profile consists 

 of two main layers: a fine-grained wind-packed layer of high density, and 

 a large-grained layer of very low density (depth hoar). Where the snow is 

 shallow the depth hoar layer may be thin or nonexistent. Although lem- 

 mings can move easily through depth hoar, dense snow may exclude 

 them from a portion of the habitat. Freezing rains or partial thaws after 



